The aim of the proposed research is to determine the passive ion permeability and regulation of volume of isolated serotonin granules as well as the intact platelet, and the effect which ion gradients and induced ion fluxes (particularly that of calcium) have upon the mechanisms of serotonin homeostasis. Particular emphasis will be placed upon the delineation of the region, regulation, bioenergetics, and physiological role of the proton gradient across the serotonin granule membrane, especially with regard to the effect upon serotonin distribution across the serotonin granule membrane and the integrity of serotonin stores. The techniques are available and will be further developed for preparation of the highest purity of isolated serotonin granules, platelet plasma membranes, and intact platelets. The approach will involve spectrophotometric, potentiometric, and radiochemical techniques which have been applied with great success to other organelle systems in the measurement of the passive ion permeability, internal pH, and regulation of volume. Ionspecific metallochromic indicators and dual wavelength spectroscopy, atomic absorption spectroscopy, and electron probe analysis in intact platelets and isolated organalles will be used to quantitate the compartmentalization of ions, particularly Ca ions, within the platelet. New nuclear magnetic resonance techniques will be employed which will allow the measurement of the pH and ATP concentration of the cytosol and serotonin granules within the intact platelet.